Introduction: MRI detection of subarachnoid hemorrhage (SAH) is an important but controversial tool for monitoring acute stroke patients. Recent publications favorably compare FLAIR (fluid attenuated inversion recovery) MRI to computed tomography (CT) for the detection of acute SAH. However, the source of this MR image contrast is not well understood. The purpose of this study was to investigate the source of the SAH contrast by comparing FLAIR images of various blood/cerebrospinal fluid (CSF) mixtures with brain. Methods: Fresh CSF was obtained from the cisterna magna and arterial blood was obtained from the femoral artery of rats. Five sample tubes were prepared: 1) heparinized blood, 2) 50/50% nonheparinized blood in phosphor buffered saline (PBS), 3) 50/50% nonheparinized blood in CSF, 4) nonheparinized blood, and 5) pure CSF. After 4 hours of settling time, the tubes were positioned next to the head of a rat and fast spin-echo FLAIR images were acquired of in-vivo brain and the sample tubes simultaneously using a 2 T GE CSI spectrometer. Transverse (T2) and longitudinal (T1) relaxation times were also measured. Results and Conclusion: All blood clots were isointense to brain on the FLAIR images. The process of clotting causes shortening of T1 and T2 to values similar to those of brain which results in little contrast on FLAIR between clots in CSF and brain. In contrast, the cells which settled in the heparinized blood were ~50% brighter than brain on FLAIR. In addition, each of the tubes containing a clot showed a thin layer of settled blood cells which was also hyperintense. T2 values for non-clotted cells were higher than for brain but less than for CSF. Hyperintensities on FLAIR images of SAH are probably due to the unclotted portion of subarachnoid blood and are best seen using long echo times.